1. Field of the Invention
The invention relates to thermoelectric switches used for the protection of electrical circuits against slowly or rapidly rising current overloads.
2. Description of the Prior Art
Conventional switches of this type contain at least one element, through which the current flows and which triggers either the switching on or the switching off function, for example a bimetal strip which changes its shape as a consequence of the Joule energy arising from the current flow, and on exceeding the predetermined maximum value triggers the switching function. For protection against very rapidly increasing current overloads, for example for short circuit protection, bimetal strips are, however, mostly unsuitable, so that for such a switching function other elements through which the current flows such as magnetic switches or fuses are necessary.
Since the discovery of the so-called shape memory alloys in the year 1961 (U.S. Pat. No. 3,012,882) it has been repeatedly suggested that these raw materials, by means of particular shape or property changes concerned with structural alterations recoverable through the influence of temperature, be used for temperature sensitive electrical switches (U.S. Pat. Nos. 3,285,470; 3,516,082 and 3,652,969 and German Patents OS-2,026,629 and 2,139,852 as well as Proceedings of the IEEE September 1970, pages 1365/66).
The completely different applications of shape memory alloys suggested in the above mentioned patents are concerned with the indirect shape change occuring suddenly and only through the influence of temperature, which will be described in the following as the one-way effect because the shape ("memory shape") prior to raising the temperature is not regained on subsequently decreasing the temperature but must first be reformed mechanically.
For thermoelectric switches whose heat sensitive element consists of a shape memory alloy, a returning device would therefore be necessary, as for example is suggested in the German Patent OS 2,139,852 for a switching element with a temperature dependent switch position comprising a combination of a NiTi shape memory alloy switching element in the form of a spring which changes its spring force at the transformation temperature with a second switching element which has a relatively temperative independent spring force. The transformation temperature of the shape memory alloy thus determines the temperature at which the switch responds so that for several switching temperatures, which according to the state of the art should lie between -50.degree. C. and +135.degree. C., different compositions are necessary in each case.
This dependence of the response temperature on the composition of a shape memory alloy used (as a switch element) according to the state of the art, exists also in those cases which are concerned with the two way effect discovered later. This function of shape memory alloys, described in more detail below, is concerned with several mechanical or thermal treatment methods and apparata such that, within certain limits, a direct purely thermal resetting of the shape changing capacity characteristics of shape memory alloys is possible (U.S. Pat. No. 3,567,523; German Patent OS -2,516,749).
The application of shape memory alloys with the two way effect in place of bimetal strips, suggested in the above mentioned patents, avoids the necessity for a device for the mechanical resetting of the memory shape condition, but has, however, the disadvantage that the respective response temperature is determined in practice by the alloy composition.
Understandably, this relation between the response temperature and the composition of the shape memory alloy, inevitable according to the state of the art, impedes practically usable technical applications, and not only because of the fact that a different alloy composition or a different heating characteristic of the triggering element of the switching process is then necessary in practice for each required switching temperature, but also because for many known shape memory alloys it is not always easy to reproduce within narrow tolerances the critical alloy composition for achieving a particular response temperature.